The present invention relates to a radiation-polymerizable mixture which is used to produce solder resist masks.
It is known to produce solder resist masks on conductor boards by a photoresist technique. Similar to the process used for the production of printed circuits, photoresist solutions are in this case deposited on patterned conductor boards, dried and cured by exposure in accordance with an image at those areas at which the conductor board is to be masked. The photoresist mask is obtained by washing out the unexposed areas of the layer. The photoresist layer can also be deposited by a dry resist process by laminating a prefabricated thermoplastic layer onto the conductor board.
Conductor boards having single-sided, double-sided, and especially plated-through construction are provided with photoresist masks before they are delivered to soldering processes. Typically, a distinction is made between two important types of mechanized soldering as follows:
1. Hot air levelling process--In this process an unassembled, plated-through conductor board produced as a pure copper circuit is formed with a solder resist mask in such a way that the copper-plated drilled holes remain free of photoresist or are uncovered. These boards are then immersed in liquid solder, the free copper elements are coated in a firmly adhering manner with the solder and then blown free with hot compressed air in the hole regions while still hot.
2. Wave or flow soldering--In this process, conductor boards having fitted components are soldered by passing them mechanically over liquid solder surfaces.
Since pure copper techniques have recently become increasingly improved and in addition, plated-through circuits are used on a wide scale, fully functional conductor boards completely fitted with components have frequently been through the two above-described soldering processes. Two solderings naturally impose high requirements on the thermal resistance of a solder resist mask. These requirements are increasingly more difficult to meet, since a number of other demands have to be satisfied.
Materials suitable for the production of solder resist masks are described, for example, in DE-A 2,747,947. These materials are photopolymerizable layers which contain a certain amount of bound halogen to improve the flame retardation. EP-A 15,004 describes a similar material which can be developed in the dry state by mechanical separation of exposed and unexposed layer regions known as the "peel-apart process". EP-A 2040 describes a photocurable material intended for the same purpose in which certain epoxy resins are used as photosensitive compounds.
In EP-B 0,113,409, a photosensitive layer is deposited over a screen printing stencil and, therefore, the photopolymerization serves only to cure the mask and is not used to produce an image.
U.S. Pat. No. 3,776,729 describes a mixture which consists essentially of photopolymerizable acrylates and thermally crosslinkable epoxides. After UV exposure to an image, a mask produced from this mixture is developed in organic solvents (for example, butanone) and cured by heating.
EP-A 0,273,729 describes a photosensitive mixture for solder resist masks, which mixture can be developed with aqueous alkali and is produced essentially by reacting epoxidized phenolic resins with acrylic acid and maleic anhydride. The reaction product does not include any epoxy groups.
According to EP-A 0,280,295, aqueous alkali development capability of an exposed mask is achieved by copolymers which include N-isobutoxymethylacrylamide and methacrylic acid as cocomponents. The solder mask obtained can be post-cured thermally, but requires relatively long curing times at relatively high temperatures.
According to DE-A 3,114,931, a bis-epoxy compound is added to a photopolymerizable mixture for producing a solder resist mask and after development, the photocured image stencil is post-cured by heating. This process produces very temperature-resistant solder resist masks. However, the mixture does not contain any inorganic pigment.
EP-A 73,444 describes similar mixtures which have a good shelf life in the unexposed state. The mixtures include compounds which can be thermally crosslinked with a binder, a polymerization product of the ethylenically-unsaturated compound and/or with themselves. Preferably, those compounds are used which include, as crosslinking groups, epoxy groups or groups of the formula --CH.sub.2 --O--R, in which R is a hydrogen atom, or a lower alkyl, acyl or hydroxyalkyl group and in which the --CH.sub.2 OR group is bonded to a nitrogen atom of a low-molecular-weight, open-chain or cyclic acid amide or to an aromatic carbon atom of a compound capable of condensing with formaldehyde.
The above-described mixtures however, have two critical disadvantages. First, under certain soldering conditions, threads or spherules of the soldering alloy continue to adhere to the surface of the solder resist mask after flow soldering, presumably as a consequence of layer softening. Under certain circumstances this may result in short circuits. As described in DE-A 3,236,560, the occurrence of these so-called "tin cobwebs" can be reduced, but not completely prevented, by roughening the photopolymer layer by means of a pigmented polyester film.
Second, the mixtures of EP-A 73,444 have an inadequate solvent resistance in the cured state. When the soldered conductor board is cleaned, softening of the solder resist masks may occur. In addition, with prolonged exposure times to solvents, for example ethanol, the dyestuff contained in the. layer is dissolved partially out of the layer.